pipexp, semi-elliptical heads are usually available in dimaters from pipe sizes up, with torispherical heads probably more available in large diameters. Readily available diameters, thicknesses, materials vary by vendor.
It all depends on what types of heads you are after. If you want hot pressed code heads then they come in standard sizes which a manufacturer can supply or you can Google it. Spun heads can be made in any size, generally up to about 7000mm diameter.
I try to work on standard hot presses sizes then it doesn't matter what they use to manufacture. Hot presses heads also have better mechanical properties plus they are cheaper and quicker to make.
If you use pipe caps to ASME B16.9 then they are classed as SE , if you check the codes it will give you the SE ratios.
Generally the shape of the head will depend on the process, in particular the pressure. Plus of course price and availability.